Generally, fine pattern formation is carried out by photolithography in the manufacture of a semiconductor device. A number of substrates called photomasks are normally used for such fine pattern formation. The photomask comprises generally a light-transmissive glass substrate having thereon a fine pattern made of a metal thin film or the like. The photolithography is used also in the manufacture of the photomask.
In the manufacture of a photomask by photolithography, use is made of a photomask blank having a thin film (e.g. a light-shielding film) for forming a transfer pattern (mask pattern) on a light-transmissive substrate such as a glass substrate. The manufacture of the photomask using the photomask blank comprises an exposure process of writing a required pattern on a resist film formed on the photomask blank, a developing process of developing the resist film to form a resist pattern in accordance with the written pattern, an etching process of etching the thin film along the resist pattern, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after writing the required pattern on the resist film formed on the photomask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using the resist pattern as a mask, an exposed portion of the thin film, where the resist pattern is not formed, is dissolved by dry etching or wet etching, thereby forming a required mask pattern on the light-transmissive substrate. In this manner, the photomask is produced.
For miniaturization of a pattern of a semiconductor device, it is necessary to shorten the wavelength of exposure light for use in photolithography in addition to miniaturization of the mask pattern of the photomask. In recent years, the wavelength of exposure light for use in the manufacture of a semiconductor device has been shortened from KrF excimer laser light (wavelength: 248 nm) to ArF excimer laser light (wavelength: 193 nm).
As a type of photomask, a halftone phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material on a light-transmissive substrate. This halftone phase shift mask is configured to have a phase shift film on a light-transmissive substrate. This phase shift film is made of, for example, a material containing a molybdenum silicide compound and is adapted to transmit light having an intensity that does not substantially contribute to exposure (e.g. 1% to 20% with respect to an exposure wavelength) and to produce a predetermined phase difference. By the use of light-semitransmissive portions formed by patterning the phase shift film and light-transmissive portions formed with no phase shift film and adapted to transmit light having an intensity that substantially contributes to exposure, the halftone phase shift mask causes the phase of the light transmitted through the light-semitransmissive portions to be substantially inverted with respect to that of the light transmitted through the light-transmissive portions so that the lights having passed near the boundaries between the light-semitransmissive portions and the light-transmissive portions and bent into the others' regions due to diffraction cancel each other out. This makes the light intensity at the boundaries approximately zero to thereby improve the contrast, i.e. the resolution, at the boundaries.
In recent years, there have also appeared a binary mask using a material containing a molybdenum silicide compound as a light-shielding film, and the like.
With respect to the photomask and the photomask blank, the miniaturization of the mask pattern of the photomask requires a reduction in thickness of the resist film formed on the photomask blank and dry etching as a patterning technique in the manufacture of the photomask.
However, the reduction in thickness of the resist film and the dry etching have the following technical problems.
One problem is that, for example, the processing time of the light-shielding film exists as one serious restriction to the reduction in thickness of the resist film on the photomask blank. Chromium is generally used as a material of the light-shielding film and, in dry etching of chromium, a mixed gas of chlorine gas and oxygen gas is used as an etching gas. When patterning the light-shielding film by dry etching using the resist pattern as a mask, since the resist film is an organic film mainly composed of carbon, it is very weak against an oxygen plasma forming a dry etching environment. While patterning the light-shielding film by dry etching, the resist pattern formed on the light-shielding film should remain with a sufficient thickness. As one index, in order to make excellent the sectional shape of the mask pattern, the resist film is required to have a thickness that still remains even when the etching time is about twice a just etching time (100% overetching). For example, since, in general, the etching selectivity of chromium as the material of the light-shielding film to the resist film is 1 or less, the thickness of the resist film is required to be twice or more that of the light-shielding film. Therefore, it is necessary to shorten the processing time of the light-shielding film for reducing the thickness of the resist film and, for that purpose, it is important to reduce the thickness of the light-shielding film.
The pattern miniaturization has advanced to require a pattern line width smaller than an exposure wavelength (ArF excimer laser light: 193 nm) so that hyper-NA exposure with a numerical aperture NA>1, for example, immersion exposure, has been developed and started to be used.
The immersion exposure is an exposure method that can improve the resolution by filling a liquid between a wafer and a lowermost lens of an exposure apparatus so that the numerical aperture is increased by the refractive index of the liquid times as compared with that in the case of air whose refractive index is 1. The numerical aperture is given by NA=n×sin θ , where θ represents an angle formed between a light ray incident on the lowermost lens of the exposure apparatus at its outermost portion and the optical axis and n represents a refractive index of a medium between a wafer and the lowermost lens of the exposure apparatus.
In this immersion exposure, an incident angle of exposure light to a photomask (an angle formed between a normal of a substrate and incident light) needs to be increased (oblique incidence). However, if this incident angle to the photomask is increased, there arises a problem of shielding effect (shadowing) which adversely affects the resolution. Specifically, when the exposure light is obliquely incident on a side wall of a pattern of the photomask, a shadow is formed due to a three-dimensional structure (particularly height) of the pattern. Because of this shadow, the size of the pattern of the photomask cannot be accurately transferred and the amount of light is reduced (less bright).
Therefore, it is necessary to reduce the height of the side wall of the pattern, i.e. the thickness of the light-shielding film.